Coordination effect-regulated CO2 capture with an alkali metal onium salts/crown ether system

Article abstract of DOI:10.1039/c3gc41513a

A coordination effect was employed to realize equimolar CO₂ absorption, adopting easily synthesized amino group containing absorbents (alkali metal onium salts). The essence of our strategy was to increase the steric hindrance of cations so as to enhance a carbamic acid pathway for CO ₂ capture. Our easily synthesized alkali metal amino acid salts or phenolates were coordinated with crown ethers, in which highly sterically hindered cations were obtained through a strong coordination effect of crown ethers with alkali metal cations. For example, a CO₂ capacity of 0.99 was attained by potassium prolinate/18-crown-6, being characterized by NMR, FT-IR, and quantum chemistry calculations to go through a carbamic acid formation pathway. The captured CO₂ can be stripped under very mild conditions (50 °C, N₂). Thus, this protocol offers an alternative for the development of technological innovation towards efficient and low energy processes for carbon capture and sequestration.

Full text of DOI:10.1039/c3gc41513a

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PAPER
Coordination effect-regulated CO2 capture with alkali metal onium
salts/crown ether system
Zhen-Zhen Yang,a, b De-en Jiang,b Xiang Zhu,b, c Chengcheng Tian,b, c Suree Brown,d Chi-Linh Do-
Thanh,d Liang-Nian He*a and Sheng Dai*b, d
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A coordination effect was employed to realize equimolar CO2 absorption, adopting easily synthesized
amino group containing absorbents (alkali metal onium salts). The essence of our strategy is to increase
the steric hindrance of cations so as to enhance a carbamic acid pathway for CO2 capture. Our easily
10 synthesized alkali metal amino acid salts or phenolates were coordinated with crown ethers, in which
highly sterically hindered cations were obtained through strong coordination effect of crown ethers with
alkali metal cations. For example, a CO2 capacity of 0.99 was attained by potassium prolinate/18ꢀcrownꢀ
6, being characterized by NMR, FTꢀIR, and quantum chemistry calculations to go through a carbamic
acid formation pathway. The captured CO2 can be stripped under very mild conditions (50 oC, N2). Thus,
15 this protocol offers an alternative for the development of technological innovation towards efficient and
low energy processes for carbon capture and sequestration.
(ILs),31ꢀ42 and aminoꢀfunctionalized solid absorbents.4, 43ꢀ49 The
traditional technology for CO2 capture is chemisorption by
50 aqueous solution of monoethanolamine (high reactivity, low cost
and good absorption capacity) to produce ammonium
bicarbonate.50, 51 However, this process for CO2 capture has some
disadvantages, including solvent loss, corrosion and particularly,
highly energy intensive owing to the thermodynamic properties
55 of water and high enthalpy of absorption.52 Notably, there are
inherent drawbacks generally associated with amino groupꢀ
containing absorbents in the absence of water, that is, the
requirement of two amine units to react with one CO2 molecule,
leading to the formation of ammonium carbamate (Scheme 1). On
60 the other hand, extensive energy input in desorption processes is
needed owing to the relatively higher stability characterization of
ammonium carbamate compared with the carbamic acid
intermediate. Hence, increasing the undesired 1:2 (CO2:amine)
stoichiometry for CO2 capacity to 1:1 and reducing the energy
65 requirement of desorption is an essential prerequisite for a
breakthrough in absorption techniques.32 Therefore, it will be
very desirable to form relatively unstable carbamic acid (1:1) for
CO2 capture, that is, to prevent the deprotonation of carbamic
acid by another amino group. In this way, equimolar CO2
70 absorption and simultaneously easy CO2 desorption will be
realized due to the relatively unstable nature of carbamic acid.
Introduction
The chemistry of CO2 has attracted much attention from the
scientific community because of the increasing atmospheric CO2
20 concentration associated with the global warming.1ꢀ9 CO2 capture
from fossil fuel combustion represents a critical component of
efforts aimed at stabilizing greenhouse gas levels in the
atmosphere.10 In addition, removal of CO2 from natural gas is of
vital importance to maintain and expand the availability of these
25 cleanꢀburning, efficient fuel sources.5
In recent years, worldwide efforts have been devoted to
developing various technologies/processes for CO2 capture,
including liquid absorbents, solid adsorbents and membranes.4, 6, 7,
9
Among them, chemisorption of CO2 based on CꢀN bond
30 formation is one of the most efficient strategies,8 utilizing liquid
absorbents such as conventional aqueous amine solutions,11ꢀ20
chilled ammonia,21ꢀ30 aminoꢀfunctionalized ionic liquids
a State Key Laboratory and Institute of Elemento-Organic Chemistry
35 Nankai University, Tianjin, 300071 (P.R. China)
E-mail: heln@nankai.edu.cn
b Chemical Sciences Division, Oak Ridge National Laboratory
Oak Ridge, TN 37831 (USA)
c State Key Laboratory of Chemical Engineering and Department of
40 Chemistry, East China University of Science and Technology, Shanghai
200237 (P. R. China)
d Department of Chemistry, University of Tennessee
Knoxville, TN 37996 (USA)
†
Electronic Supplementary Information (ESI) available: General
45 experimental methods, synthetic procedure of alkali metal onium salts,
and characterization of the absorption systems (NMR, TGA and IR). See
DOI: 10.1039/b000000x/
Scheme 1 Reactions of CO2 with amino groupꢀcontaining absorbents.
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Table 1 CO2 absorption using crown ether/alkali metal onium salts
mixtures (molar ratio 1:1)a
As readily accessible chiral molecules, natural αꢀamino acids
(AAs) have many merits, such as diverse structures, low cost,
high thermal stability, bioꢀcompatibility, easy regeneration and
biodegradability.53 Phosphonium and ammonium ionic liquids
(ILs) with AAs as anions have been already used for CO2 uptake,
but the CO2:AA stoichiometry arising from the formation of
ammonium carbamate is still 1:2.34ꢀ38 In this respect, specially
designed absorbents derived from AAs have been successfully
synthesized and applied for 1:1 CO2 capture through a carbamic
5
10 acid formation pathway. Gurkan et al. have synthesized two AAꢀ
based ILs with a longꢀchain alkyl phosphonium cation, including
trihexyl(tetradecyl) phosphonium prolinate([P66614][Pro]) and
methioninate ([P66614][Met]), both of which can react with CO2 in
a ratio of one CO2 per amine (1:1 stoichiometry) by termination
15 of the reaction sequence at the formation of carbamic acid.33 We
also designed AA salts with bulky Nꢀsubstituents for
chemisorption of CO2, approaching almost equimolar absorption
in polyethylene glycol (PEG) solution.54 The key idea of these
two successful examples leading to the formation of carbamic
20 acid is highly steric hindrance, either derived from bulky cations
[P66614]+, or bulky Nꢀsubstituents (iPr), to prevent nucleophilic
attack of the amino group to the carboxylic acid group of
carbamic acid. Despite such great advances, the development of
efficient processes for CO2 capture with simple, easily prepared
25 absorbents is still appealing. Therefore, it is desirable to develop
a way to make easily synthesized AA salts with “small” cations
and regular anions, such as commercially available alkali metal
cations and AA anions, to obtain an equimolar absorption of CO2
Entry
1
2
3
4
Absorbent
18ꢀcrownꢀ6
15ꢀcrownꢀ5
Timeb (min)
CO2 capacityc
0.04
5
5
5
25
10
ꢀ
30
10
20
20
15
15
10
15
5
15
10
30
10
30
0.03
0.03
0.89
0.99
~0.90
0.83
0.79
0.75
0.75
0.73
0.71
0.68
0.63
0.75
0.84
0.90
0.85
12ꢀcrownꢀ4
ProNa/15ꢀcrownꢀ5
ProK/18ꢀcrownꢀ6
[P66614][Pro]
5
6d
7e
ProK/PEG300
8
9
ValK/18ꢀcrownꢀ6
MetK/18ꢀcrownꢀ6
GlyK/18ꢀcrownꢀ6
ThrK/18ꢀcrownꢀ6
AlaK/18ꢀcrownꢀ6
LeuK/18ꢀcrownꢀ6
SerK/18ꢀcrownꢀ6
PhOLi/12ꢀcrownꢀ4
PhONa/15ꢀcrownꢀ5
PhOK/18ꢀcrownꢀ6
[P66614][PhO]
10
11
12
13
14
15
16
17
18f
19
20f
through
a carbamic acid formation pathway. Moreover,
30 carboxylate group of the AA salts has the potential to stabilize the
formed carbamic acid through intramolecular hydrogenꢀbond
formation.
4ꢀClPhOK/18ꢀcrownꢀ6
[P66614][4ꢀClꢀPhO]
0.95
0.82
Crown ethers are cyclic chemical compounds that consist of
several ether groups, which can strongly bind certain cations to
35 form complexes, probably possessing some toxicity to creatures55,
thus having the potential to form bulky cations.56ꢀ63 Herein, we
found that the coordination effect between crown ether and alkali
metal cations (increasing steric hindrance) leads to an extremely
high CO2 capacity, approaching almost equimolar absorption in
40 PEG solution, adopting easily synthesized onium salts derived
from AAs or phenols and alkali metal hydroxide as absorbents.
Coordination effectꢀadjusted CO2 absorption is assumed to
proceed via the carbamic acid formation rather than the
subsequent ammonium carbamate. On the other hand, the
45 coordination effect leading to the formation of relatively unstable
carbamic acid, greatly reduces the energy requirement for CO2
desorption, in comparison with ILs with the same anions.
a Absorption conditions: crown ether/alkali metal onium salts (molar ratio
60 1:1) 1 mmol; PEG300, 2 mmol; CO2, 0.1 MPa; 25 oC. b Time required to
reach absorption equilibrium. c Moles of CO2 captured per mole of onium
salt, was estimated from the weight increase of the reaction mixture. d
Data from Ref. 33 e 0.2643 g PEG300 (the same weight as 1 mmol 18ꢀ
.
crownꢀ6) was added instead of 18ꢀcrownꢀ6. f Data from Ref. 67
.
65 physical interaction was observed (entries 1ꢀ3). For various alkali
metal cations, the corresponding crown ethers on the basis of size
fit within the interior of the macrocycle is needed to for effective
coordination (18ꢀcrownꢀ6 for K+, 15ꢀcrownꢀ5 for Na+, 12ꢀcrownꢀ
4 for Li+),58 and the ionic diameter for cations or diameter of
70 cavity for crown ethers is 1.20 (Li+), 1.90 (Na+), 2.66 (K+), 1.2ꢀ
1.5 (12ꢀcrownꢀ4), 1.7ꢀ2.2 (15ꢀcrownꢀ5) and 2.6ꢀ3.2 (18ꢀcrownꢀ6),
respectively.62 Alkali metal cations together with corresponding
crown ethers, the CO2 capacity of which increases in the order of
Na+/15ꢀcrownꢀ5 < K+/18ꢀcrownꢀ6 for ProM, and Li+/12ꢀcrownꢀ4
75 < Na+/15ꢀcrownꢀ5 < K+/18ꢀcrownꢀ6 for PhOM, could play a key
role in CO2 sorption performance (entries 4ꢀ5, 18ꢀ20). In
particular, coordination of ProK with 18ꢀcrownꢀ6 greatly
enhanced the CO2 capacity, approaching the 1:1 stoichiometry
(CO2 capacity of 0.99), which is consistent with the theoretical
80 formation of carbamic acid, comparable with the IL [P66614][Pro]
with the same anion (~0.90) (entry 5 vs. 6).33 However, inferiority
was existed for our systems (e.g. ProK/18ꢀcrownꢀ6/PEG300) by
valuing mass of CO2 absorbed per gram of the absorbent, owing
to addition of large quantities of PEG300 as solvent. In fact, the
85 solvent PEG300 itself was able to coordinate with the cation K+
Results and Discussion
Easily synthesized alkali metal AA salts and phenolate (onium
50 salts) together with different crown ethers were investigated to
test our idea about coordinationꢀregulated CO2 absorption (Table
1). PEG300 (M.W. = 300 Da) was selected as a suitable solvent
due to its distinctive properties, such as low cost, almost
negligible vapor pressure, toxicological innocuity and CO2ꢀphilic
55 property.64ꢀ66 As shown in Table 1, crown ether/PEG300 gave
extremely poor CO2 sorption capacity, implying that only
2
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DOI: 10.1039/C3GC41513A
in ProK,54, 68 although not as strong as 18ꢀcrownꢀ6, and gave a
100
90
80
70
60
50
40
30
20
10
0
(a)
CO2 capacity of 0.83 (entry 7). However, for other potassium AA
onium salts, a CO2 capacity of 0.63ꢀ0.79 was attained, probably
due to the high viscosity of the system after CO2 absorption, but
still much higher than the corresponding ILs with the same AA
anions (~0.50) through an ammonium carbamate formation
pathway.37
ProK/18-crown-
6/PEG300+CO2
18-crown-6
5
PEG300
Interaction between CO2 (carbon atom) and ether linkages
(oxygen atom) in both PEG and crown ethers leads to lowered
10 viscosity and increased gas/liquid diffusion rates, and thus
assumedly facilitates CO2 sorption with faster sorption kinetics
and increasing capacity.38 Indeed, a CO2 capacity of 0.90 was
obtained within 10 min adopting PhOK/18ꢀcrownꢀ6, while 30
min was needed for [P66614][PhOK] to get a comparable CO2
15 capacity (0.85) (entry 17 vs. 18). In addition, 4ꢀClꢀPhOK/18ꢀ
crownꢀ6 gave a much better performance for CO2 capture (CO2
capacity of 0.95 in 10 min), than the corresponding IL with the
same anion [P66614][4ꢀClꢀPhOK] (CO2 capacity of 0.82 in 30 min)
(entry 19 vs. 20).
Extensive energy input in the desorption process would be a
crucial barrier to realize practical CO2 capture, and also stability
and reusability of the absorbent are two important factors to
identify whether it finds potentially practical application in
industry. The stability of ProK/18ꢀcrownꢀ6/PEG300 after CO2
0
100
200
300
400
500
Temperature/oC
(b)
1
0.8
0.6
0.4
0.2
0
20
25 capture was studied by thermogravimetric analysis (TGA) (Fig.
1(a)). It can be seen that the thermal stability of the absorbent
system ProK/18ꢀcrownꢀ6/PEG300 was increased compared with
18ꢀcrownꢀ6, which can also account for the coordination effect
between 18ꢀcrownꢀ6 and K+. Notably, the captured CO2 was easy
30 to be stripped out just at a temperature as low as 50 oC by
bubbling N2 within 25 min. Since higher temperatures are
required for CO2 removal from the common conventional amineꢀ
0
50
100
Time/min
150
200
250
60
Fig. 1 (a) The scanning thermogravimetric analysis (TGA) results for 18ꢀ
crownꢀ6, PEG300 and ProK/18ꢀcrownꢀ6/PEG300+CO2 with a 10 oC minꢀ1
temperature ramping rate to 500 oC. (b) Six consecutive cycles of CO2
absorption (♦ 15 min, 25 oC, CO2 0.1 MPa) and release (◊ 25 min, 50 oC,
65 N2 0.1 MPa) by ProK/18ꢀcrownꢀ6/PEG300 system, data were obtained
every 5 min.
o
based absorbents (e.g. 120 C, N2, 75 min),69 this can prove that
our system proceeds through the carbamic acid pathway (Scheme
35 2). Notably, no significant drop in CO2 capacity was detected
after six successive absorptionꢀdesorption cycles (Fig. 1(b)). For
the 4ꢀClꢀPhOK/18ꢀcrownꢀ6/PEG300 system, the absorbed CO2
ppm to 174.7 ppm, which was quite different from the 13C NMR
spectrum of carbamic acid (Fig. 2(b)). For the 4ꢀClꢀPhOK/18ꢀ
crownꢀ6 system, coordination between 18ꢀcrownꢀ6 and K+ can
70 also be detected by 1H NMR (Fig. S1(a)). After CO2 capture, all
the protons of 4ꢀClꢀPhOꢀ showed a downfield shift and the new
peak in 13C NMR spectrum at 158.7 ppm could prove the
formation of phenylcarbonate [PhOCO2]ꢀ (Fig. S1(b)).
In the in situ FTꢀIR spectrum, there are three important
75 features. Firstly, the strength of OꢀH stretching from PEG
increased after CO2 absorption due to formation of carbamic acid
(OꢀH containing group), and buleshifted from 3298 cmꢀ1 to 3472
cmꢀ1, also indicating that PEG could be chemically inert during
the process (Fig. 3(a)). Secondly, distinct bands corresponding to
80 the ammonium cation between 2000ꢀ3000 cmꢀ1 region are not
observed, while the new peak at 2338 cmꢀ1 derived from
physically dissolved CO2. Finally, the original vibration of the
C=O bond of the carbonyl group in Proꢀ was shown at 1598 cmꢀ1,
and then after CO2 uptake, two characteristic peaks centered at
85 1635 cmꢀ1 and 1667 cmꢀ1 can be assigned to asymmetric C=O
vibrations of the carboxylate anion and COOH moiety of the
carbamic acid, respectively (Fig. 3(b)). For the 4ꢀClꢀPhOK/18ꢀ
crownꢀ6/PEG300 system, the carbonyl group absorption was
o
can be removed completely at 80 C by bubbling N2 within 20
o
min, much more easily than IL [P66614][4ꢀClꢀPhO] (80 C, N2, 60
40 min) (Fig. S2(b)).
To further gain insight into the absorption mechanism,
coordinationꢀregulated absorption involving the formation of
carbamic acid (Scheme 2) was identified by NMR and FTꢀIR. It
1
was found that in the H NMR spectrum all the protons of Proꢀ
45 and 18ꢀcrownꢀ6 (H1ꢀH5) showed an upfield shift after mixing
them together, due to the coordination of 18ꢀcrownꢀ6 with K+
cation (Fig. 2(a)). Afterwards, the downfield shift of all the
proton in Proꢀ (H2ꢀH5) after CO2 absorption could indicate that
CO2 is chemically bound to the nitrogen center (Fig. 2(a)).
50 Simultaneously, the new peak in 13C NMR spectrum at 157.7
ppm after CO2 uptake would support the formation of the
carbamic acid structure between the secondary amine and CO2
(C7), while C6 of the carbonyl group in Proꢀ showed an upfield
shift from 177.7 ppm to 175.9 ppm after CO2 absorption (Fig.
55 2(b)). Notably, bicarbonate anion being formed from reaction
between CO2 and aqueous (D2O) solution of Prok, showed a new
peak at 160.2 ppm, while C6 of the carbonyl group in ammonium
cation (protonated Proꢀ) resulted in an upfield shift from 177.7
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0.6
(a)
ProK/18-crown-6/PEG300
ProK/18-crown-6/PEG300+CO2
0.4
0.2
3298 cm-1
3472 cm-1
Scheme 2 Reaction of CO2 with ProK/18ꢀcrownꢀ6 to produce carbamic
acid with PEG300 as solvent.
0
(a)
1
600
1100 1600 2100 2600 3100 3600
Wave number/cm-1
18-crown-6
3
5
2
ProK
0.4
4
1598 cm-1
(b)
1
18-crown-6/ProK
1635 cm-1
1667 cm-1
3
5
4
2
0.2
18-crown-6/ProK/PEG300 + CO2
3
0
2
5
4
1501 1601 1701
ProK/18-crown-6/PEG300
(b)
6
ProK/18-crown-6/PEG300 + CO2
7
30 Fig. 3 Results of in situ IR spectroscopy under 0.1 MPa CO2 pressure
monitoring at various reaction time. (a) ProK/18ꢀcrownꢀ6/PEG300 (molar
ratio 1: 1: 2) before and after CO2 uptake at 25 oC; (b) Magnifying threeꢀ
dimensional spectra of the absorption mixture with reaction time.
6
ProK + CO2 in D2O
Å
5
Fig. 2 (a) 1H NMR spectra (CDCl3, 300 MHz) of 18ꢀcrownꢀ6, ProK,
ProK/18ꢀcrownꢀ6 and ProK/18ꢀcrownꢀ6/PEG300+CO2 (from top to
bottom); (b) 13C NMR (CDCl3, 75 MHz) spectra of ProK/18ꢀcrownꢀ
6/PEG300 before (top) and after (middle) CO2 absorption, and 13C NMR
10 (D2O, 75 MHz) spectra of CO2 capture by D2O solution of ProK (bottom).
35 Scheme 3 The quantum chemistry calculations of the reaction between
CO2 and ProK/18ꢀcrownꢀ6 in a PEG300 solution to produce carbamic
acid; the PEG300 is modelled as a continuum medium with a dielectric
constant of 13.4. H: gray, C: black, N: pink, O: blue, K+: yellow.
observed at 1647 cmꢀ1 after CO2 capture, indicating the formation
of phenylcarbonate [PhOCO2]ꢀ (Fig. S1(c)).
Quantum chemistry calculations, being performed with the
program Turbomole V6.0,70 were employed to calculate the
15 enthalpy changes for the present equimolar CO2 capture (Scheme
3). The PEG solvent was simulated as a continuum medium with
Conclusions
40 In summary, a coordination effect was employed to realize
equimolar CO2 absorption, adopting easily synthesized amino
group containing absorbents (alkali metal onium salts), and the
essence is to increase the steric hindrance of cations and go
through a carbamic acid pathway. Our strategy is to utilize easily
45 synthesized alkali metal AA salts together with crown ethers, in
which highly sterically hindered cations were obtained through
strong coordination effect of crown ethers with alkali metal
cations. For example, a CO2 capacity of 0.99 was attained by
ProK/18ꢀcrownꢀ6, being detected by NMR, FTꢀIR and quantum
50 chemistry calculation to go through a carbamic acid formation
pathway. The captured CO2 can be stripped out under very mild
a
dielectric constant of 13.471 within the Conductorꢀlike
Screening Model (COSMO)72 implemented in Turbomole. Both
structure optimization and the solvation calculation were
20 performed at the B3LYP level with the def2ꢀTZVP basis sets.73
The amino acid salt ProK with 18ꢀcrownꢀ6 coordinated at the K+,
reacts with CO2 at the secondary amine to form the carbamic acid,
which is stabilized through an intramolecular hydrogen bond with
the carboxylate anion (bond length: 1.593 Å). In addition,
25 formation of the carbamic acid has a calculated enthalpy change
of ꢀ39.0 kJ/mol, indicating that the absorption process is
thermodynamically favourable, but not too strong. This is
consistent with the easy desorption we have found (Fig. 1b).
o
conditions (50 C, N2). In addition, crown ethers together with
alkali metal phenolates are also easily synthesized absorbents for
efficient CO2 absorption. Thus, this protocol would pave the way
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and low energy demanding practical processes for CO2 capture.
65
70
Acknowledgements
This research was sponsored by the Division of Chemical
Sciences, Geosciences, and Biosciences, Office of Basic Energy
Sciences, U.S. Department of Energy.
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